The present invention is generally related to radio transceivers and more particularly related to an improved method and a corresponding circuit to adjust the frequency of an oscillator which controls the frequency synthesis within a receiver circuit.
With regard to cost reduction, low cost crystal oscillators instead of expensive temperature compensated crystal oscillators (TCXO) are used to generate the frequency of a reference oscillator for transceivers. In the particular case of usage of none temperature compensated crystal oscillators frequency accuracy improvement in general with additional circuitry is necessary to compensate the frequency deviation of the oscillator.
In the prior art methods are known to control the frequency of a reference oscillator by counting the zero crossings of a received and downconverted signal for a predefined time. The average number of zero crossings to be expected is known in such systems. The deviation of the expected number of zero crossings to the counted number of zero crossings will be calculated and a corresponding output value is generated. This digital output value is given to a Digital to Analogue Converter and the resulting analogue signal is used to control the frequency of the reference oscillator.
A known method to control the frequency synthesis of the reference oscillator is described in NO 172418 as follows.
After downconversion and hard limiting a received signal will be given to a counter that counts the number of zero crossings for a predefined time interval. The length of this time interval is a multiple of the system clock cycle that is derived from the voltage controlled crystal oscillator (VCO) frequency via a Phase Locked Loop circuit (PLL). The Master Control Unit (MCU) of the device computes the frequency deviation of the voltage controlled crystal oscillator (VCO) from the difference between counted and expected number of zero crossings and generates an output value. A digital to analogue converter (DAC) converts this value into a corresponding analogue voltage that controls the frequency of the crystal oscillator. The expected number of zero crossings is related to the nominal, unmodulated intermediate frequency of the downconverted signal.
The counted number of zero crossings depends on the actual intermediate frequency that is generated from the received Radio Frequency Signal by means of the mixer frequencies. Since the mixer frequencies are derived from the crystal frequency, the difference between the counted and expected number of zero crossings is an indicator for the frequency deviation of the crystal.
If, for example, a system with .pi./4 DQPSK modulation is used, the instantaneous frequency depends on the transmitted data. In order to eliminate the influence of the modulation counting for a relatively long time interval is necessary to obtain a usable value to control the frequency of the reference oscillator.
For the case that the received signal contains non random data or bits representing only 0's or 1's in the data bit stream this method fails because the expected number of zero crossings can only be based on a signal where the bits are well balanced like in random data fields. In fact the presently used data transmission methods contain partly non random and random data.
Therefore it is an object of the present invention to obtain an automatic frequency control algorithm which eliminates the disadvantages described above.